Heretofore, it has been well known to provide snap elements for electrical switching operation. Dome-shaped snap elements for switches, such as disclosed in U.S. Pat. Nos. 3,710,209; 3,751,612; 3,967,084; 4,029,916; and 4,083,100, are particularly well known. Each of these patents shows a snap-acting element that is dome-shaped and which may have one or more feet. These patents also show it is old to use plural elements in keyboards. Further, these snap-acting elements are monostable in that they will only take one position when not subjected to any force and whereby once depressed by a force they will return to the original position when the force is released. Obtaining appropriate tactile feel in these elements is always a problem.
The dome snap-acting elements known heretofore are formed of relatively thin stock and are formed in a dome-shaped configuration whereby they collapse upon being depressed and then return to their original shape once the collapsing force has been removed.
Many of the dome-shaped snap-acting elements heretofore known are more particularly in the form of a rigid dome-shaped structure when generally becomes increasingly stiff or inelastic as the dome diameter is decreased for a given material thickness. Accordingly, the characteristics of such a dome snap-acting element are not particularly suitable for miniaturization of switching elements, which is a trend in the industry.
Many keyboards are designed to have a flush face where no visible keys can be discerned and where only a flat surface with printed key indicia is visible and accessible to the operator. Where dome switches are used in such configurations, it is often necessary to additionally include secondary snap means to enhance the tactile feel. Such secondary means causes additional expense in the manufacture of the keyboard. Minimal tactile feel is particularly apparent in small size dome snap elements where the thickness of the material of the dome is so thin that no meaningful tactile feel can be experienced. Increasing the thickness of the material tends to cause a loss of elasticity and snap action. This is particularly noticeable as the size of the dome switches decrease in diameter.
It is also well known that dome snap-acting elements are not considered to be true overcenter snap elements. A true overcenter snap device is bistable, wherein it must snap from one stable position to another stable position. Thus, conventional dome snap-acting elements, being monostable, cannot serve as bistable elements. Only a momentary function such as closing of an electrical circuit can be produced by such dome switches. For example, in a thermal switch that has opened an electrical circuit due to malfunction caused by overheating, it is desirable to maintain that open position unil the problem causing the malfunction has been corrected. Thus, dome snap-acting elements cannot properly serve as a bistable switch element.
It is also known that dome snap-acting elements cannot oppose more than a few ounces of force to a snapping-over action as the material forming the dome must be very thin so that it can acquire acceptable snap-acting characteristics.
It is known that the environment under which some keyboards are used require the use of gloves which may insulate completely against a snap-acting tactile feel of heretofore known dome switch elements, that thereby decrease their usefulness.
Dome snap-acting elements heretofore known must be mechanically deformed and stressed in order to acquire the desired snap-acting properties. Precise deformation of the material for these elements is difficult to obtain, thereby limiting the control over the degree and uniformity of stressing and ultimate tactile feel characteristics. Further, major tooling costs are required for each change in size of dome snap-acting elements and desired snap force.
Prior known small bimetallic snap discs are unable to snap and operate in response to small temperature differentials, thereby limiting their use as a low differential thermal control device. Indeed, snap-acting elements of heretofore known types are essentially stiff and rigid in construction and have very little useful elasticity. They therefore do not function well, if at all, in light of small changes in ambient temperatures.
It is sometimes necessary to provide "buffer" circuits to properly condition the momentary electrical pulse resulting from the depressing of heretofore known snap-acting dome switches. The need for such circuits is caused by the normal differential in human depressing forces to a key structure whereby the electrical pulse generated may be of different durations.